This article is from the Hiking/Backpacking With Canines, by Terri Watson with numerous contributions by others.
Most working breeds and athletic mid-sized dogs can carry about 33% of
their body weight. Start the dog off with 25% of their weight for a
short hike and see how they do. Gradually work up to 33% and
progressively longer hikes. If you have a small (eg, Fox Terrier) or
large breed (eg, Great Dane), or a breed with a less athletic build,
then your dog's ideal pack weight may be a bit different from the
percentages listed here.